Catalytic reforming, or hydroforming, is a well established industrial process employed by the petroleum industry for improving the octane quality of naphthas or straight run gasolines. In reforming, a multi-functional catalyst is employed which contains a metal hydrogenation-dehydrogenation (hydrogen transfer) component, or components, substantially atomically dispersed upon the surface of a porous, inorganic oxide support, notably alumina. Platinum metal catalysts, or platinum to which one or more additional metal promoters have been added to form polymetallic catalysts are currently employed, reforming being defined as the total effect of the molecular changes, or hydrocarbon reactions, produced by dehydrogenation of cyclohexanes and dehydroisomerization of alkylcyclopentanes to yield aromatics; dehydrogenation of paraffins to yield olefins; dehydrocyclization of paraffins and olefins to yield aromatics; isomerization of n-paraffins; isomerization of alkylcycloparaffins to yield cyclohexanes; isomerization of substituted aromatics; and hydrocracking of paraffins which produces gas, and inevitably coke, the latter being deposited on the catalyst.
In a reforming operation, one or a series of reactors, providing a series of reaction zones, are employed. Typically, a series of reactors are employed, e.g., three of four reactors, these constituting the heart of the reforming unit. Each reforming reactor is generally provided with a fixed bed, or beds, of the catalyst, each receives down-flow feed, and each is provided with a preheater or interstage heater, because the reactions which take place are endothermic. A naphtha feed, with hydrogen, is cocurrently passed through a preheat furnace and reactor, and then in sequence through subsequent interstage heaters and reactors of the series. The product from the last reactor is separated into a C.sub.5 + liquid fraction which is recovered, and a vaporous effluent. The vaporous effluent is a gas rich in hydrogen, and usually contains small amounts of normally gaseous hydrocarbons, from which hydrogen is separated and recycled to the process to minimize coke production.
The sum-total of the reforming reactions occurs as a continuum between the first and last reaction zone of the series, i.e., as the feed enters and passes over the first fixed catalyst bed of the first reactor and exits from the last fixed catalyst bed of the last reactor of the series. During an on-oil run, the activity of the catalyst gradually declines due to the build-up of coke on the catalyst, and hence during operation, the temperature of the process is gradually raised to compensate for the activity loss caused by the coke deposition. Eventually, however, economics dictate the necessity of reactivating the catalyst. Consequently, in all processing of this type the catalyst must necessarily be periodically regenerated by burning off the coke in the presence of an oxygen-containing gas at controlled conditions. Catalyst reactivation is then completed in a sequence of steps wherein the agglomerated metal hydrogenation-dehydrogenation components are atomically redispersed.
Regeneration processes are basically of two types. In a semi-regenerative process, the entire unit is operated by gradually and progressively increasing the temperature to maintain the activity of the catalyst, caused by the coke deposition, until finally the entire unit is shut down for regeneration, and reactivation, of the catalyst. After regeneration, and reactivation of the catalyst the unit is put back on-oil. In a cyclic regeneration process, the reactors are individually isolated, or in effect swung out of line by various manifolding arrangements, motor operated valving and the like. The catalyst is regenerated to remove the coke deposits, and then reactivated while the other reactors of the series remain on stream. A "swing reactor" temporarily replaces a reactor which is removed from the series for regeneration and reactivation of the catalyst, until it is put back in series. The cyclic method of regeneration offers advantages over the semi-regenerative type process in that the catalyst, because it can be continuously regenerated, and reactivated, without shutting down the unit, suffers no loss in production. Moreover, because of this advantage the unit can be operated at higher severities to produce higher C.sub.5.sup.+ liquid volume yields of high octane gasoline than semi-regenerative reforming units.
In reforming, there is a contemporary need for a high activity, high yield, sulfur tolerant, low coke make catalyst. High yields of aromatics rich reformate relieve the yield/octane pressure arising from lead removal. High activity relieves reactor volume-limited refineries and compensates for reduced refinery capacity by permitting higher naphtha feed throughput. Greater sulfur tolerance permits relaxation of feed hydrofining severities, this providing process energy and hydrogen savings. Low coke formation extends the frequency of the regeneration, or permits shorter regeneration periods, or both. Thus, economic credits are provided by energy savings, or shorter off-oil intervals.
Catalysts constituted of platinum and iridium, with or without the presence of an additional promoter metal, or metals, are known to provide this combination of properties. Platinum-iridium catalysts are the most active of commercial reforming catalysts. For example, the ability of iridium to promote platinum activity provides catalytic activities two to four times that of the more conventional platinum and the now widely used platinum-rhenium catalysts, respectively, depending upon the platinum and iridium loadings. Unfortunately however, platinum-iridium catalysts unlike the platinum and platinum-rhenium catalysts all too readily agglomerate and inactivate upon exposure to oxygen at high temperatures. For this reason the wide application of platinum-iridium catalysts in commercial operations has been restricted, especially to exclude their use in cyclic reforming units, since time-consuming and inefficient regeneration procedures are required to avoid damaging the iridium. Known methods of regeneration thus utilize lengthy, low temperature coke burns in the presence of a chloride-rich environment to maintain the iridium in highly dispersed state. Low oxygen concentrations in the combustion gas are also employed during the combustion period to hold the flame front temperature below about 800.degree. F. (426.7.degree. C.). The use of chloride during this prolonged burning period in itself creates a number of troublesome problems. For example, the use of scrubbing equipment is required to remove the corrosive chloride containing gases from the gas recycle stream. Moreover, volatile iron chlorides are formed by reaction of the chlorine with reactor walls, and the deposition of these iron salts on the reforming catalyst contributes to poor on-oil performance. Despite the admirable properties of platinum-iridium catalysts for on-oil use, the tendency of the iridium component of the catalyst to agglomerate on regeneration thus imposes a major liability upon the use of these catalysts; especially, as relates to their use in cyclic regeneration operations.